I’m working on a new stepper motor controller sensor for the Camera Axe. As you cans see I’ve decided to go for a really full featured stepper controller and put controls for 3 stepper motors and a microcontroller on the sensor. The camera axe will talk to this board via the IC2 protocol. This was the best way I could figure out to control more than 2 stepper motors with the camera axe, and even if you only need two stepper motors (three is really nice for some uses cases) this design has advantages. Some of those are being able to power off the stepper motors to save power, and using less program space/cpu cycles in the Camera Axe’s main controller. The only disadvantage is a slightly more expensive sensor board.

There are a lot of use cases such as:

Panoramic and/or rail timelapse

Gigapixel images

Interfacing with a microscope to do focus stacking and micro-gigapixel type images

Focus stacking and gigapixel type images for macro images

Lots more

The biggest unknown I have about this board is if I really need a separate 5V power source instead of taking it from the batteries powering the motors. I have some experience of this sort of setup (same power source) working fine, but I have read that sometimes it can lead to flakiness. I wonder if that’s true or if people didn’t put enough filter caps in their design.

If anyone wants to help with the mechanical designs for some of these use cases let me know.

3 Comments

Morten Thillemann said,

Maybe I can help with the mechanics, being mech. engineer and building machinery every day.
A suggestion to getting 5 V would be to get some of these cheap 12V-5V dc-dc devices for USB-in-car.
They provide up to 1.5A. For a conveinient power source, a 7AH sealed battery or maybe a NiMh battery from some power tool, as they also are easier to carry.
What weight should you expect for the camera, 3kg?
How many frames are requiered for quality stitching, (angular precision/gear/zeroing movement)

Maurice Ribble said,

I’ve changed the design a bit and removed the 5V regulator. Instead I’m drawing 5V directly from the Camera Axe so that the 12V is only used to run motors and things like voltage spikes won’t reset the micro.

I’m planning to use a stepper like this: http://www.sparkfun.com/products/9238 So the motors run directly off 12V, but they don’t have that much torque. I think we’ll need to gear them down a bit. The other advantage of gearing them down is we’ll get more than 200 steps per rotation. I’m pretty sure if we can get 1000 or more steps we should have more than enough resolution for very fine detailed stitching. In a forum post another use case of astrophotography came up. http://www.dreamingrobots.com/forums/viewtopic.php?f=8&t=168

I think 3 kg for a camera would be a good max. Most cameras will be in the 1-2 kg range.

Feel free to email me (ribblem ‘at’ yahoo.com) if you want to work with me on this.

My main goals is to make an inexpensive solution that people can make for themselves or buy from the Camera Axe store. The tricky thing is keeping the price down.

Basically, what they do is take a low-voltage stepper motor, such as 3V, and feed it off 12V or 24V. Since the motor is inductive, the current doesn’t rise to V/R immediately. The Driver uses a sense resistor in series with the motor to monitor the current, and when the motor current reaches a certain threshold, it switches the motor to ground, and waits for the current to decay back down to a second threshold, and repeats the cycle.

The end result is that the stepper motor is basically being used as the inductor in a buck DC-DC converter where it is also the load.

The advantage is that the significantly higher supply voltage means the current rise time is much faster, so you can drive the stepper faster. Also, since the motor is shorted a portion of the time (the current decay period), there is some improvement in stall torque.

The biggest advantage, I think however, is that the drivers have internal state-machines which handle motor coil sequencing, so all you need to control the motor is step and direction lines.